Electron discharge device and associated circuit



July 20, 1948. B. J. MAYO 4 2,445,404

ELECTRON DISCHARGE DEVICE AND ASSOCIATED GIRCUIT Filed March 13, 1944INVETOR Behm-J "are Amer Patented July 20, 1948 ELECTRON DISCHARGEDEVICE AND ASSOCIATED CIRCUIT Bernard Joseph Mayo, Hayes, England,assignor to Electric & Musical Industries Limited, Hayes, England, acompany of Great Britain Application March 13, 1944, Serial No.

In Great Britain August 8, 1941 Section 1, Public Law 690, August s,1946 Patent expires August 8, 1961 This invention relates to electrondischarge devices employing hollow resonators constructed for use asgenerators of oscillations. The invention is concerned with such devicesof the kind in which reflecting means are provided capable, if suitablycharged, of setting up a reflecting field to cause the electrons in theelectron beam of the device after having passed through the gap in theresonator where they become velocity modulated to be reflected backthrough the gap. The resonator in such devices may be of toroidal formand is tuned in known manner to the frequency of the oscillations thatit is desired to generate.

It is necessary for efficient operation that the electric field which isset up in operation between the resonator and the reflecting electrodeshould firstly be such as to cause a reasonable degree of bunching.Secondly, the field should be such as to ensure that as much as possibleof the electron beam is reflected back through the gap and thirdly thefield should be such that the drift time of the axial and marginalelectrons is substantially the same so that substantially all suchelectrons after reflection reach the gap in phase.

It has been proposed in known forms of devices of the kind referred to,to employ a reflecting electrode in the form of a flat disc or in theform of a, portion of a sphere with the concavity facing the resonator.In the former case the electric field which is set up between theresonator and the reflecting electrode is composed of equipotentialsurfaces which, adjacent the resonator, follow the contour of the latterand facing the resonator are convex at the aperture in the resonator,but which, as they recede from the resonator, rapidly become flatter andthereafter remain flat throughout the remaining distance. Where thereflecting electrode comprises a portion of a sphere as aforesaid, hereagain the equipotential surfaces adjacent the resonator follow thecontour of the resonator and are convex at the aperture but subsequentlythe equipotential surfaces become flat and then become concave facingthe resonator with increasing curvature as they recede from theresonator, the equipotential surface having the maximum curvature beingthe one closest to the reflecting electrode, the curvature of suchsurface being the same as the curvature of the reflecting electrode.Since the reflecting electrode is maintained at a negative potentialwith respect to the potential of the cathode, the 'zero equipotentialsurface, i. e., the surface having the 4 Claims. (01. 250-275) potentialof the cathode, is situated between the resonator and the reflectingelectrode. Where the reflecting electrode is in the form of a flat discthe equipotential surfaces between the zero equipotential surface andthe reflecting electrode are all flat, whilst where the reflectingelectrode is of partially spherical form as aforesaid, the equipotentialsurfaces between the zero equipotential surface and the reflectingelectrode are of increasing curvature.

A flat reflecting electrode is satisfactory in that it affords efficientbunching, but where, as is usually the case, the electron beam isdivergent as it passes towards the reflecting electrode, very few of themarginal electrons will be reflected back through the aperture intheresonator. Hence the device will lack efliciency.

A system of concave equipotential surfaces as set up by a concavereflecting electrode is eflicient in its ability to reflect both theaxial and marginal electrons back through the resonator, but isineflicient in that only a small degree of bunching of the electronsoccurs.

It will be appreciated therefore that devices embodying either a flat orconcave reflecting electrode do not satisfy the aforementionedrequirements for efl'icient operation.

I have found that these requirements can be more completely satisfiedcompared with known constructions of devices of the kind referred to ifthe equipotential surfaces which are set up in operation transversely tothe path of the electron beam are of concave form facing the resonator,the curvature of said surfaces as they recede from the resonatorincreasing as they approach the zero equipotential surface and in theregion of the central part of the beam decreasing as they recede fromthe zero equipotential surface.

According therefore to one feature of the invention, an electrondischarge device of the kind referred to is provided in which saidreflecting means is so constructed as to be capable, if suitablycharged, of setting up equipotential surfaces to cause reflection of thebeam, which surfaces transversely to the path of the beam, are ofconcave form facing the resonator, the curvature of said surfaces asthey recede from the resonator increasing as they approach the zeroequipotential surface and in the region of the central part of the beamdecreasing as they recede from the zero equipotential surface so as toimprove the eiflciency of the device. As stated above, the equipotentialsurfaces immediately adjacent the resonator will be convex owing tothose surfaces which lie between the point where the surfaces change toconcave form and the reflecting means. beam is, for the purpose of thespecificat n,.,1d e fined as that part of the beam lying along thecentre of the space occupiedl fby. theb mland.

which has a cross-section approximatelyfequaij to half the area of theaperture from whichthe beam emerges from the resonator.

According to another feature of theinvention.

there is provided a circuit arrangement ,e

bodying an electron discharge devicelof the referred to in which saidreflecting means is so constructed and is maintained at suchapotem tialwith respect to the potential of the resonator nd atn ss of eaidw e i eit et u he derer tentia sur-ie esr fe dzt abov so .t ammate e f e en r,th euce P ef bl aid refle ti -m ans qmr s e e e in ectr de und b aShort Qq dnc n ur ageh h;rem ra es h eam.

h en ion imar a appl dect 0 dishar e de is s m ler ns :31? rqn b am. hamsa u stan ia y i ula or-m rbss..-se i9 in which casethe.reflectingelectrode is of'cirler form h v n r hera dense and also toele tm i e a edeyicesi wh ch t a e t beam -;is of ribbon-shape in which.ca e thereflecting electrode will be of elongatedform to it hebb n-s aed beam- 11 ord ha h aidin ent on ma be clearly understoodand-readilycarried into effect, it will now bemore fully-- described with referenceto the m any n d win in whi Figure-1 illustratesdiagrammatically anelec-i tron discharge device according to the; pref erred for of the-invention, ,and

Fi ure 2 is a diaeramen n enlareefiz al the equipotential surfaces whichare-set up. in pe ation etween the. e ona r a d, h r flec ingelectrodeof thedevice, shown in Figure 1, Figure 3 is asectiontakenalongTthe line ,3,.. -3 of Figure; 1; Figure 4 is, a sectiontaken along a similar line as Fi ure 3'; but showing a modification of"the apertures and reflecting electrode.

As ,shown-in Figure 1, a cathode is provided for generatinga beam ofelectrons, the, cathode being surrounded-by apathodeshield 2'. Afocussing electrode "3 isprovided in the form of an apertured diaphragmwhich serves, in conjunction with-the cathodeshield 2, to generate, whenthe electrodes are maintained at suitable potentials, a. focussingfield. The ,hollowresonator isindicated by the reference numeral 6; andis of toroidal shape andprovidedwith a pair of oppositely-disposedapertures 5 and .B-providing a gap through which the; electronbeam iscaused to pass. The field, which serves to reflect the electron beamback through the aperture 6- in the resonator for thepurpose ofmaintaining the. generation ofoscillations in the resonator, is :set upby the provision of. a reflecting electrode 1 which, in operation, issuitably chargedby maintaining it at a negative potential-with respectto the cathode l.

The device shown in the drawing employs a beam ofcircular formincross-section and it is desirable that the electronstream should forma cross-over at the gap. Consequently, the electron;beam, when itemergesfrom the, resonator,

Also the ntral rest f, 11s.-

4 will be of divergent form. The reflecting electrode I, since theelectron beam has a circular cross-section, is of circular form and inorder to improve the field in the path of the beam so that therequirements aforesaid are more completel satisfied, the electrode I issubstantially flat and is provided with a short conducting surfaceenbracing the beam, said conducting surface being in the form of aperipheral flange m as show j-Fig'ui'efZ of the drawings illustratessome of the equipotential surfaces which are set up between theresonator t and the reflecting electrode =I whenthelatter is maintainedat a negative potential-withzrespect to the cathode l and when theresonator 4 is maintained at a positive potential with respect to thecathode I. As shown,

' the e'quipotential surfaces at and b are of convex form inthe vicinityof the aperture 6 in the resonator whilst between the equipotentialsurface b and the surface c shown in the figure the equipotentialsurfaces change from the convexform,

to the concave form, as shown. The curvature of the equipotentialsurfaces as exemplified .by the surface d increases as they approach thezero equipotential surface which may be the surface e and said surfacesin the region of the central part of the beam as they recede from thezero equipotential surface decrease in curvature as will b seen from theshape of the surface 1. The divergence of the beam in its passagebetween the resonator i and the reflecting electrode 1 may be asindicated by the dotted lines shown in Figure 2 and it will be observedthat whilst the decreas ing curvature mentioned above holds for thecentral part of the beam the curvature in the region of the marginalelectrons of the beam is greater compared with the curvature of the saidsurfaces in the region of the centralpart of the beam. The equipotentialsurfaces which lie between the surfaces ;f and the reflecting electrodel, as exemplified by the surface g further decrease in curvature andthose surfaces which lie close. to the reflecting electrode .will followthe contour of said electrode. The electrons will however be reflectedbefore they reach the reflecting electrode. Thus, by providing thereflecting electrode with ashort conducting surface which embraces thebeam, a field can be set up between the resonator and-the reflectingelectrode which is effectively a compromise of'the fields which would beproduced when using a flat reflecting electrode without a conductingsurface embracing the beam and when employing a reflecting electrode ofconcave form. The equipotential-surfaces will of course extend beyondthe limits shown in Figure 2 and the portions beyond the limits shownwill, where the surfaces are close to the reflecting electrode, tend tolie substantially parallel to the flange la. These portions, however,play substantially no part in the re,- flecting operation since they donot lie in the path of the beam.

It is. found that there is an optimum ratio of diameter of theelectrodeI to the width of the flange M for maximum efficiency. This optimumratio is approximately 3, that is to say, the width of-the flange 1a isone-third of thediameter of the disc. It is also found that there is. anoptimum diameter for the reflecting electrode of approximately three tofour times the diameter of the aperture 6 in the resonator. The aperture6, in one example, may be 2 mms, and with such a diameter the diameterof the reflecting elec rod isan qximai fi to 6 .1

The invention can also be applied to electron discharge devices whichemploy ribbon-shaped beams as aforesaid. In such a case the apertures inthe resonator are elongated to accommodate the ribbon-shaped beam andthe reflecting electrode is of elongated form suitable for causingreflection of the ribbon-shaped beam. The reflecting electrode in thiscase will be provided with a short conducting surface embracing thebeam, but such surface need not extend around the whole periphery ofsaid electrode but may merely extend along its longer sides. The optimumdimensions referred to above also hold for the elongated form ofreflecting electrode, but in this case the ratios referred to are inrespect of the minor axes of the reflecting electrode and the elongatedaperture.

It is preferred to dispose the reflecting electrode close to theresonator and since, in operation, the resonator will be maintained at ahigh positive potential and the reflecting electrode at a low ornegative potential, there is the danger that, due to the proximity ofthe reflecting electrode to the resonator, sparking there-between mayoccur. In order to reduce this possibility the wall of the resonatoradjacent the reflecting electrode 1 is provided with an annulardepression 8, as shown, in the vicinity of the peripheral flange Ia ofthe reflecting electrode so that an adequate space is aflorded betweenthe periphery of the flange and the adjacent surface of the resonator toreduce the possibility of sparking.

In operation of the device shown in the drawing the focussing electrode3 may be maintained at a potential of 300 to +300 volts with the cathodeI and the cathode shield 2 at zero potential, the resonator is at apositive potential of 1000 to 2000 volts and the reflecting electrode 1at a negativ potential of 150 to 500 volts. Figure 3 shows a crosssection of Figure 1 taken along the line 33 showing an end view of thedevice utilizing my invention in which a disc electrode arrangement isused. Figure 4 is a view similar to Figure 3, showing a modificationutilizing an elongated slot 60 and elongated reflecting electrode 1chaving opposed flanges 1c directed toward the cavity resonator 4.

Figures 3 and 4 show end views of a device utilizing my invention inwhich Figure 3 shows the disc electrode arrangement 1 and Figure 4 theelongated slot and the elongated reflecting electrode 10.

I claim:

1. An electron discharge device having a cathode for supplying a streamof electrons, and a reflecting electrode in the path of said electrons,a cavity resonator having a pair of oppositely disposed aperturesthrough which said stream path extends, the transverse dimension of theaperture in said resonator adjacent said refiecting electrode beingbetween one-third and onefourth the transverse dimension of saidreflecting electrode, said reflecting electrode having a conductingflange parallel to the beam path, said flange having a dimension alongthe beam path one-third that of the transverse dimension of saidreflecting electrode.

2. An electron discharge device having a cathode for supplying a streamof electrons and a reflecting electrode in the path of said electrons,said reflecting electrode comprising a disc, a cavity resonatorpositioned between said cathode and said reflecting electrode and havingoppositely disposed circular apertures in the walls thereof throughwhich the path of said stream of electrons lies, said apertures having adiameter between one-third and one-fourth the diameter of saidreflecting electrode, said reflecting electrode having a flangeextending toward said resonator and having a dimension along the path ofsaid electrons equal to one-third the diameter of said reflectingelectrode.

3. An electron discharge device having a cathode for supplying a streamof electrons, a reflecting electrode in the path of said electrons, acavity resonator positioned between said cathode and said reflectingelectrode, said cavity resonator having oppositely disposed aperturesthrough which the path of the stream of electrons lies, said reflectingelectrode being elongated and said apertures being elongated, theshorter transverse axis of the reflecting electrode being between threeand four times the shorter transverse axis of said apertures, saidreflecting electrode having a flange extendingtoward said resonator andhaving a dimension parallel to the path of the electron stream equal toonethird the shorter transverse axis of said reflecting electrode.

4. An electron discharge device having a cathode for supplying a streamof electrons and a reflecting electrode in the path of said electrons,and a cavity resonator positioned between said cathode and saidreflecting electrode, and hav ing apertures in the walls thereof throughwhich the electron path lies, said reflecting electrode having a flangeextending toward said resonator and lying parallel to the path of theelectrons, said resonator and said reflecting electrode being closelyspaced, said resonator having a depression registering with the flangeon said reflecting electrode.

BERNARD JOSEPH MAYO.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 2,190,511 Cage Feb. 13, 19402,190,515 Hahn Feb. 13, 1940 2,190,735 Rust Feb. 20, 1940 2,244,672Brett June 10, 1941 2,250,511 Varian et al. July 29, 1941 2,259,690Hansen et a1. Oct. 21, 1941 2,280,026 Brown Apr. 14, 1942 2,293,151Linder Aug. 18, 1942

